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<title>Improved integration with C arrays and strings</title>
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<div class="document" id="improved-integration-with-c-arrays-and-strings">
<h1 class="title">Improved integration with C arrays and strings</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr><th class="docinfo-name">Author:</th>
<td>Thorsten Ottosen</td></tr>
<tr><th class="docinfo-name">Contact:</th>
<td><a class="first last reference" href="mailto:thorsten.ottosen&#64;dezide.com">thorsten.ottosen&#64;dezide.com</a></td></tr>
<tr class="field"><th class="docinfo-name">Organizations:</th><td class="field-body"><a class="reference" href="http://www.dezide.com">Dezide Aps</a></td>
</tr>
<tr><th class="docinfo-name">Date:</th>
<td>2007-03-08</td></tr>
<tr class="field"><th class="docinfo-name">Number:</th><td class="field-body">WG21/N2225 and J16/07-0085</td>
</tr>
<tr class="field"><th class="docinfo-name">Working Group:</th><td class="field-body">Library</td>
</tr>
</tbody>
</table>
<div class="contents topic">
<p class="topic-title first"><a id="table-of-contents" name="table-of-contents">Table of Contents</a></p>
<ul class="auto-toc simple">
<li><a class="reference" href="#introduction" id="id1" name="id1">1&nbsp;&nbsp;&nbsp;Introduction</a></li>
<li><a class="reference" href="#wording" id="id2" name="id2">2&nbsp;&nbsp;&nbsp;Wording</a><ul class="auto-toc">
<li><a class="reference" href="#synopsis-for-basic-string" id="id3" name="id3">2.1&nbsp;&nbsp;&nbsp;Synopsis for <tt class="docutils literal"><span class="pre">basic_string</span></tt></a></li>
<li><a class="reference" href="#specification-for-basic-string" id="id4" name="id4">2.2&nbsp;&nbsp;&nbsp;Specification for <tt class="docutils literal"><span class="pre">basic_string</span></tt></a></li>
<li><a class="reference" href="#synopsis-for-vector" id="id5" name="id5">2.3&nbsp;&nbsp;&nbsp;Synopsis for <tt class="docutils literal"><span class="pre">vector</span></tt></a></li>
<li><a class="reference" href="#specification-for-vector" id="id6" name="id6">2.4&nbsp;&nbsp;&nbsp;Specification for <tt class="docutils literal"><span class="pre">vector</span></tt></a></li>
</ul>
</li>
</ul>
</div>
<div class="section">
<h1><a class="toc-backref" href="#id1" id="introduction" name="introduction">1&nbsp;&nbsp;&nbsp;Introduction</a></h1>
<p>Quite a few libraries start out as a C library and is then
wrapped in a thing C++ layer for convenience and type-safety.
However, standard class templates <tt class="docutils literal"><span class="pre">std::vector</span></tt> and <tt class="docutils literal"><span class="pre">std::basic_string</span></tt>
does not allow us to seamlessly wrap a C API without great loss of efficiency.
This is very <em>embarrassing</em>.</p>
<p><tt class="docutils literal"><span class="pre">std::basic_string</span></tt> has already paid the price for C compatibility by storing 
a terminating null, thus allowing us to get a C compatible string with <tt class="docutils literal"><span class="pre">basic_string::c_str()</span></tt>.
However, the compatibility is one way: from C++ to C. When going from C to C++ we are forced
to <em>copy</em> the entire buffer.</p>
<p>The same annoyance comes up with heap-allocated arrays: because we are forced to
<em>copy</em> a C array into an <tt class="docutils literal"><span class="pre">std::vector</span></tt> object, we pay a huge price performance wise if we want to
upgrade legacy code to a more maintainable style.</p>
<p>The fix is easy: add two new member functions to <tt class="docutils literal"><span class="pre">std::vector</span></tt> and <tt class="docutils literal"><span class="pre">std::basic_string</span></tt> 
that gives us explicit control over the buffer. This example shows how to
take ownership of a C string, manipulate it with the C++ interface, and then give up ownership again:</p>
<pre class="literal-block">
//
// Define a compatible string
//
typedef std::basic_string&lt;char,malloc_allocator&lt;char&gt;&gt; String

//
// Step 1: acquire buffer
//
char* str = get_c_string();
String new_string;
new_string.acquire_buffer( str, std::strlen( str ) );    
assert( str == 0 );

//
// Step 2: manipulate buffer
//
new_string.replace( ... );

//
// Step 3: give up ownership
//
str = new_string.release_buffer();
assert( new_string.empty() );
free( str );
</pre>
<p>In addition to improving interoperability with C, <em>legacy C++ code will also
benefit</em> because we can almost directly replace <tt class="docutils literal"><span class="pre">new</span></tt>'ed arrays/strings with
<tt class="docutils literal"><span class="pre">std::vector</span></tt>/<tt class="docutils literal"><span class="pre">std::string</span></tt>.</p>
</div>
<div class="section">
<h1><a class="toc-backref" href="#id2" id="wording" name="wording">2&nbsp;&nbsp;&nbsp;Wording</a></h1>
<div class="section">
<h2><a class="toc-backref" href="#id3" id="synopsis-for-basic-string" name="synopsis-for-basic-string">2.1&nbsp;&nbsp;&nbsp;Synopsis for <tt class="docutils literal docutils literal"><span class="pre">basic_string</span></tt></a></h2>
<p><em>Add the following members to the synopsis of</em> <tt class="docutils literal"><span class="pre">basic_string</span></tt>:</p>
<pre class="literal-block">
void    acquire_buffer( charT*&amp; newBuffer, size_type length, const Allocator&amp; a );
charT*  release_buffer();
</pre>
</div>
<div class="section">
<h2><a class="toc-backref" href="#id4" id="specification-for-basic-string" name="specification-for-basic-string">2.2&nbsp;&nbsp;&nbsp;Specification for <tt class="docutils literal docutils literal"><span class="pre">basic_string</span></tt></a></h2>
<p><em>Add the following specification after the synopsis of</em> <tt class="docutils literal"><span class="pre">basic_string</span></tt>:</p>
<p><tt class="docutils literal"><span class="pre">void</span> <span class="pre">acquire_buffer(</span> <span class="pre">charT*&amp;</span> <span class="pre">newBuffer,</span> <span class="pre">size_type</span> <span class="pre">length,</span> <span class="pre">const</span> <span class="pre">Allocator&amp;</span> <span class="pre">a</span> <span class="pre">=</span> <span class="pre">Allocator()</span> <span class="pre">);</span></tt></p>
<ul class="simple">
<li><em>Preconditions</em>: <tt class="docutils literal"><span class="pre">newBuffer</span></tt> is a null-terminated buffer of length <tt class="docutils literal"><span class="pre">length</span></tt> (not counting null) . <tt class="docutils literal"><span class="pre">a.deallocate()</span></tt> can 
be used to reclaim the memory of the buffer.</li>
<li><em>Effects</em>: deallocates the current buffer, copies <tt class="docutils literal"><span class="pre">a</span></tt> into the container's allocator and sets <tt class="docutils literal"><span class="pre">newBuffer</span></tt>
as the new buffer.</li>
<li><em>Postconditions</em>: <tt class="docutils literal"><span class="pre">size()</span> <span class="pre">==</span> <span class="pre">length</span> <span class="pre">&amp;&amp;</span> <span class="pre">size()</span> <span class="pre">==</span> <span class="pre">capacity()</span> <span class="pre">&amp;&amp;</span> <span class="pre">newBuffer</span> <span class="pre">==</span> <span class="pre">0</span></tt></li>
<li><em>Remarks</em>: if an exception is thrown, <tt class="docutils literal"><span class="pre">newBuffer</span></tt> is deallocated with <tt class="docutils literal"><span class="pre">a.deallocate(newBuffer,length+1)</span></tt>.</li>
</ul>
<p><tt class="docutils literal"><span class="pre">charT*</span> <span class="pre">release_buffer();</span></tt></p>
<ul class="simple">
<li><em>Effects</em>: releases ownership of the internal buffer and returns it.</li>
<li><em>Postconditions</em>: <tt class="docutils literal"><span class="pre">empty()</span></tt></li>
<li><em>Throws</em>: Nothing</li>
<li><em>Remarks</em>: <tt class="docutils literal"><span class="pre">get_allocator()</span></tt> may be used to obtain an allocator that can deallocate the buffer.</li>
</ul>
</div>
<div class="section">
<h2><a class="toc-backref" href="#id5" id="synopsis-for-vector" name="synopsis-for-vector">2.3&nbsp;&nbsp;&nbsp;Synopsis for <tt class="docutils literal docutils literal"><span class="pre">vector</span></tt></a></h2>
<p><em>Add the following members to the synopsis of</em> <tt class="docutils literal"><span class="pre">vector</span></tt>:</p>
<pre class="literal-block">
void  acquire_buffer( T*&amp; newBuffer, size_type length, const Allocator&amp; a = Allocator() );
T*    release_buffer();
</pre>
</div>
<div class="section">
<h2><a class="toc-backref" href="#id6" id="specification-for-vector" name="specification-for-vector">2.4&nbsp;&nbsp;&nbsp;Specification for <tt class="docutils literal docutils literal"><span class="pre">vector</span></tt></a></h2>
<p><em>Add the following specification after the synopsis of</em> <tt class="docutils literal"><span class="pre">vector</span></tt>:</p>
<p><tt class="docutils literal"><span class="pre">void</span> <span class="pre">acquire_buffer(</span> <span class="pre">T*&amp;</span> <span class="pre">newBuffer,</span> <span class="pre">size_type</span> <span class="pre">length,</span> <span class="pre">const</span> <span class="pre">Allocator&amp;</span> <span class="pre">a</span> <span class="pre">=</span> <span class="pre">Allocator()</span> <span class="pre">);</span></tt></p>
<ul class="simple">
<li><em>Preconditions</em>: <tt class="docutils literal"><span class="pre">newBuffer</span></tt> is a buffer of length <tt class="docutils literal"><span class="pre">length</span></tt>. <tt class="docutils literal"><span class="pre">a.deallocate()</span></tt> can 
be used to reclaim the memory of the buffer.</li>
<li><em>Effects</em>: deallocates the current buffer, copies <tt class="docutils literal"><span class="pre">a</span></tt> into the container's allocator and sets <tt class="docutils literal"><span class="pre">newBuffer</span></tt>
as the new buffer.</li>
<li><em>Postconditions</em>: <tt class="docutils literal"><span class="pre">size()</span> <span class="pre">==</span> <span class="pre">length</span> <span class="pre">&amp;&amp;</span> <span class="pre">size()</span> <span class="pre">==</span> <span class="pre">capacity()</span> <span class="pre">&amp;&amp;</span> <span class="pre">newBuffer</span> <span class="pre">==</span> <span class="pre">0</span></tt></li>
<li><em>Remarks</em>: if an exception is thrown, <tt class="docutils literal"><span class="pre">newBuffer</span></tt> is deallocated with <tt class="docutils literal"><span class="pre">a.deallocate(newBuffer,length)</span></tt>.</li>
</ul>
<p><tt class="docutils literal"><span class="pre">T*</span> <span class="pre">release_buffer();</span></tt></p>
<ul class="simple">
<li><em>Effects</em>: releases ownership of the internal buffer and returns it.</li>
<li><em>Postconditions</em>: <tt class="docutils literal"><span class="pre">empty()</span></tt></li>
<li><em>Throws</em>: Nothing</li>
<li><em>Remarks</em>: <tt class="docutils literal"><span class="pre">get_allocator()</span></tt> may be used to obtain an allocator that can deallocate the buffer.</li>
</ul>
</div>
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